We study the subnanosecond optical addressing capabilities of semiconductors based on the diffraction properties of plasma index gratings. We use degenerate four-wave mixing experiments at 1.06 micrometers . The diffraction efficiency of the refractive index grating remains constant ((eta) equals 5 X 10^-4) in the nanosecond time scale. The grating erasure is possible however using a uniform picosecond illumination: an 80% decrease of the diffraction efficiency is achieved with only 25% of the writing energy. We show that the best model to explain our experimental results is based on the plasma density modulation stabilized by charged deep centers via the electrostatic screening effect.

A novel flat-panel display made of hydrogenated amorphous silicon nitride and silicon carbide Thin Film LED (TFLED) has been developed. The a-SiN:H and a-SiC:H were prepared by a glow discharge plasma CVD technique. The emission colors can be obtained from red, yellow, green to white-blue. The amorphous TFLED can be fabricated as a large area on any foreign substrate. Any emission pattern can be obtained by designing the pattern of the internal thin film electrodes. A series of technical data on the preparation of the a-SiN:H/a-S:C:H TFLED is presented. Some new ideas of the utilizations of the developed TFLED in optoelectronic functional devices are presented and described.

A time dependent model describing near travelling wave optical amplifiers is presented. This model is used to design and optimize semiconductor optical amplifiers used as fast external modulators with internal gain.

We demonstrate picosecond signal recovery in all optical gate operation using a type II tunneling bi-quantum well (TBQ) etalon. Type II TBQ consists of a series of GaAs wells, AlGaAs barriers, and AlAs layers. The recovery time from excitonic absorption bleaching in GaAs wells is governed by tunneling of electrons out of the well through AlGaAs barriers into AlAs layers. The type-II TBQ etalon with 1.7 nm barriers shows 17 ps-recovery.

In this paper we describe a fiber optic system which was used for information reading from the F_A center temporary memory. The fiber optic system was used to replace the probe wave of the spectrophotometer used in earlier studies.

An optoelectronic memory cell was constructed from two photoconducting (PC) and two electroluminescent (EL) devices. PC and EL devices were prepared by vacuum evaporation. This optoelectronic memory cell was supplied with a sinusoidal voltage. The input signal was in the shape of rectangular light pulses illuminating the PC device. The output signal was in the form of the light emitted from the electroluminescent device. Applied PC and EL devices within the optoelectronic memory cell ought to perform the determined condition, namely the optical feedback between them must exist. A value of the feedback coefficient ought to be such as to perform the bi-stable circuit operation.

An `ultra clean' atmospheric pressure chemical vapor phase deposition (APCVD) system has been used to grow SiGe/Si quantum well structures on planar and patterned substrates. The optical properties have been analyzed by photoluminescence (PL) and the effect of growth temperature, well width, and Ge concentration on the PL properties has been studied. Phonon resolved spectra were obtained for growth temperatures exceeding 600 degree(s)C for selective and non-selective growth. Spectra taken from selectively grown samples reveal additional lines related to dislocations and twin formation. Twinning was found predominantly along the (110) direction of the SiO₂/Si interface, whereas interfaces along the (100) direction had a much lower defect density. Temperature resolved measurements indicate a free exciton like behavior for the no-phonon (NP) line of the SiGe quantum wells. Time resolved measurements show decay times of the SiGe NP-line in the range of 0.5 to 0.8 microsecond(s) , comparable to those found for the Si TO phonon line. This demonstrates the capability of APCVD to grow selectively and non-selectively SiGe/Si quantum well structures with low defect densities.

The optoelectronic properties of ultrathin Si_mGe_n strained layer superlattices (SLSs) depend strongly on their structural perfection and the strain adjustment of the SLS by a Si_1-xGe_x alloy buffer. We used double crystal and triple axis x-ray diffractometry to characterize the structural properties of short period Si₆Ge₄ and Si₉Ge₆ SLSs grown on about 1 micrometers thick step-graded SiGe alloy buffers. As grown SLSs and samples annealed subsequently at 550 degree(s)C, 650 degree(s)C, and 780 degree(s)C for 60 min were investigated, the latter to study effects of post-growth thermal treatments typical for conventional Si device fabrication. Precise strain data were extracted from two-dimensional reciprocal space maps around (004) and (224) reciprocal lattice points. These data were used as refined input parameters for the dynamical simulation of the integrated intensity along the q[004] direction.

We report time-resolved fluorescence measurements in 6H-SiC samples in a subnanosecond time range with a high spatial resolution. The fluorescence kinetics of the 6H-SiC substrate grown by the Lely-method is characterized by a mono-exponential decay with a time constant of about 50 ns. The epilayer of 5 micrometers thickness grown by liquid phase epitaxy shows a significant faster decay in the subnanosecond range which can be explained by a distinguished higher defect concentration in comparison to the substrate. The spatially resolved detection demonstrates that the lifetime change within 100 micrometers . The red wavelength range shows a slower fluorescence lifetime than the blue one.

In the present paper we discuss the electronic properties of semiconductor heterojunctions, focusing on the band offset problem. We address interface-specific phenomena, where the conditions of growth -- including controlled contamination and strain effects -- may significantly alter the properties of the junction. We study the effects of ultrathin intralayers (i.e., heterovalent implantation) both at homojunctions (such as GaAs/Ge/GaAs) and heterojunctions (such as GaAs/Si/AlAs). In both cases, our theory demonstrates how the intralayers control the band offset: this result is confirmed by recent experimental observation in several systems. We then consider the band-offset engineering at lattice-mismatched heterojunctions, studying the paradigmatic case GaAs/Si. We evaluate the band-offset for several fully relaxed inequivalent interfaces. Both macroscopic strain and microscopic morphology effects are considered.

We summarize here a systematic investigation of the structural and electronic properties of III- V/IV semiconductor heterojunctions (with IV equals Si, Ge, and III-V equals GaAs, AlAs) as well as III-V/IV/III-V single and multiple quantum well structures. All structures were fabricated by molecular beam epitaxy and characterized in-situ by reflection high energy electron diffraction and x-ray photoemission spectroscopy, and ex-situ by x-ray diffraction and transmission electron microscopy. We found that relatively abrupt composition profiles and ideal pseudomorphic growth could be achieved through appropriate deposition conditions. Measurements of the band discontinuities indicated that large deviations from the commutativity and transitivity rules of heterojunction band offsets are observed in most of these interfaces. Such deviations demonstrate the dependence of the band discontinuities on the local interface environment and are related, in general, to the establishment of inequivalent local interface environments.

We present the shadow mask molecular beam epitaxial (MBE) growth technique which allows an in situ lateral structuring of the doping profile and the growth rate on a micrometers scale. The electrical dc characteristics show that excellent quality selective contacts have been achieved on devices with lateral dimensions down to the micrometers range. The leakage currents are, even for highly doped structures, in the nA range. High reflectivity Bragg mirrors and pronounced exciton peaks observed on MQW structures confirm the high quality of the regrown material. The influence of the aspect ratio on the growth rates is very small. We have applied this novel technique to fabricate various selectively contacted optoelectronic devices based on n-i-p-i doping superlattices. For GaAs Franz Keldysh n-i-p-i modulators with selective contacts an on/off ratio of 6:1 has been achieved. High frequency results obtained on medium size devices indicate that 3 dB frequencies in the GHz range should be possible for n-i-p-i devices with dimensions < 4 micrometers fabricated with this technique. By selectively contacting the QWs in a modulation doped hetero n-i-p-i structure constructive superposition of field and carrier induced absorption changes have been achieved.

In this paper we report an in-depth study of the nonlinear optical properties associated with intersubband transitions in AlInAs/GaInAs coupled quantum wells grown by MBE. Structures with giant nonlinear susceptibilities (Chi) ⁽²)(2 (omega) ) and (Chi) ⁽³)(3 (omega) ) (compared to the bulk constituents of the quantum wells) have been designed and demonstrated. They exhibit large linear Stark shifts of the intersubband transitions which have been used to efficiently tune the nonlinear susceptibilities. The second order nonlinear susceptibility (Chi) ⁽²)(2 (omega) ) exhibits a peak as a function of the electric field corresponding to the energy levels being made equally spaced via the Stark effect. In a three-coupled-well structure triply resonant third harmonic generation has been observed. The corresponding (Chi) _{(3 (omega} ))⁽³) (10^-14(m/V)² at 300 K and 4 X 10^-14 (m/V)² at 30 K) is the highest measured in any material. The equivalent of multiphoton ionization of a molecule has also been investigated in this structure.

We report full scale calculations with relativistic pseudopotentials of first and second order optical susceptibilities in p-type GaAs-AlAs and Si-SiGe quantum well structures in which the response arises due to excitations between valence minibands. Our results contain several novel features. In particular, the peak value of the frequency dependent susceptibility is controlled by optical transitions originating from regions of the momentum space lying further from the center of the Brillouin zone. The excitation steps that give a quantitative meaning to the optical nonlinearity involve quantum states lying above the semiclassical barrier.

We produced high quality single crystalline ZnTe/ZnSe/GaAs (001) waveguides by atmospheric pressure metalorganic-vapor-phase epitaxy (MOVPE). Using end-fire coupling we observed a strong non-phase-matched second harmonic of the 1319 nm Nd:YAG fundamental (30 W peak power). In order to find out the best geometry for the second harmonic generation (SHG) in the ZnTe waveguide we measured the polarization dependence of this nonlinear process. Concepts of phase matched SHG in ZnTe waveguides are discussed.

A very large second-order susceptibility (Chi) ⁽²)(2(omega) ), responsible for frequency doubling, is obtained for interband transitions at near-infrared wavelengths in a step-like asymmetric quantum well based on GaSb-InAsSb compounds. The structure is engineered so as to achieve a double energy resonance between the electron and hole states and to maximize all relevant dipole matrix elements. The optimized structure has (Chi) ⁽²)(2(omega) ) approximately equals 9 X 10^-9 m/V at a pump wavelength of 1.5 micrometers , with an enhancement of about 30 times with respect to the second-order susceptibility of bulk GaAs.

A density matrix approach has been used to describe fast nonlinear phenomena, such as carrier heating and spectral hole burning, in semiconductor laser devices. The aim of this paper is to investigate highly non degenerate frequency mixing in semiconductor lasers amplifiers and oscillators. Coupled mode approach is used to analyze intermodulation in travelling wave laser amplifiers.

We report on opto-electrical and opto-optical bistability with high electrical/optical gain occurring at very low optical power. In p-i-n and n-i-p-i structures large field induced changes of the absorption coefficient up to 5000 cm^-1 can be observed due to the Franz- Keldysh effect. Slightly above the bandgap the absorption decreases with increasing internal field. This leads to n-shape current voltage characteristics which can be used for bistable opto- electrical switching. At an optical power smaller than 200 pW we achieve an on/off ratio of more than 10⁷ for switching the n-layer current corresponding to an opto-electrical gain of 6.8 (DOT) 10⁶. A smart pixel consisting of an electro-optical n-i-p-i modulator controlled by the bistable switch allows the observation of opto-optical bistability. With our smart pixel concept we have achieved an opto-optical gain of up to 3 (DOT) 10⁵. This bistable switching is observed at extremely low switching powers smaller than 500 pW and exhibits a very broad hysteresis width of 200 pW. The minimum switching power is only limited by the dark current of the switch and can be reduced further.

We report both linear and nonlinear grating coupling to a planar multiple quantum well (MQW) waveguide grown by chemical beam epitaxy. We demonstrate a strong dependence of the coupling angle on the incident power in the wavelength range (lambda) equals 1.57 - 1.60 micrometers . The obtained nonlinear variation of n_eff were found to be negative, and on the order of 10^-3, in agreement with many-body calculations performed by the authors. We show that thermal effects can be ruled out. At coupled light intensity as low as 40 mW, nonlinear switching condition was achieved; the power density to obtain the switching process is substantially lower than in previous works. As part of this work, we report the systematic investigation of the linear and nonlinear absorption properties of several InGaAs/InP MQW structures, measuring a series of transmission spectra in the region of the band edge for various pump intensities and observing large changes in the absorption coefficient. Through comparison with these results, we speculate that the mechanism responsible for the refractive nonlinearity observed in the waveguide is consistent with purely excitonic effects.

Transmission spectra recovery of CdSe, CdS_xSe_1-x nanocrystals and porous silicon wires optically excited by ultrashort laser pulses have been studied with picosecond time resolution using pump and probe technique. The transitions between levels of electrons and holes spatially confined within nanocrystals and thin wires were observed as bleaching bands in nonlinear transmission spectra (saturation effect). Spectra and values of third order resonant susceptibility were determined for nanocrystals of different size and porous silicon using the experimentally measured differential transmission spectra.

We report on the MBE growth of nominally 2 and 3 monolayers (ML) thick InAs quantum wells embedded in GaAs. The structures presented were grown using different substrate temperatures and growth interruption times. They were characterized by high resolution x-ray diffraction and low temperature photoluminescence (PL) measurements. The PL spectrum for the 3 ML sample was dominated by a strong electron-hole pair recombination at 1.262 eV, with a full width at half maximum (FWHM) of 39 meV. For a nominally 2 ML InAs quantum well the main PL peak was shifted to higher energies. The PL emission from a second 2 ML thick film having a growth interruption at the InAs/GaAs interface exhibited a large shift to lower energies and a broader linewidth. The evolution from 2 to 3 dimensional growth mode is reflected in the PL spectra and the x-ray diffraction patterns. The spectral positions of the PL peaks are in good agreement with estimates of the subband energies from a 3-band Kane model including the effects of strain.

The effects of elastic strain induced by lattice mismatch on the effective mass and band off-set at (Gamma) point are studied in this paper. We found that the effective masses (electron, light and heavy hole) become anisotropic in a strained layer. The theoretical calculations are derived using the method of linear combination of atomic orbitals (LCAO) and the overlapping integrals are adjusted in accordance with the change of atomic distance. For unstrained and strained interfaces, the band offset ratios of 90:10 and 57:43 (conduction band:valence band) are obtained, respectively. Calculating the strained In_XGa_1-XSb structures with our calculated results, the excellent agreements are found between theoretical calculations and experimental results in this study, thus the parameters of our calculations are suitable for the calculations of the strained systems.

The effect of traps due to lattice mismatch between GaAs and InP materials on the reverse current of Schottky diodes is demonstrated by the temperature dependence of the current, which exhibits a S.R.H. component at low reverse bias (also present in GaAs/GaAs with activation energy 0.125 eV) and a trap assisted tunneling one at high reverse bias (not observed in GaAs/GaAs). A model is developed which takes into account the temperature and channel doping level dependence. Application of this model to 0.25 micrometers gate GaAs MESFETs gives a good agreement with gate leakage current behavior as a function of drain and gate bias, for 6 X 10¹⁷ cm^-3 and 10¹⁸ cm^-3 channel doping. The excess gate-drain assisted tunneling current in 10¹⁸ cm^-3 doped channel does not affect the MESFETs dc and microwave performances. However, the microwave noise (Fmin) is increased.

The band offset at semiconductor heterojunctions is shown to be extremely sensitive to the presence at the interface of a delta-doping layer with sheet density much smaller than that of a full monolayer. Different models are analyzed in the effective-mass approximation in order to cover the full range of densities, from the low-density isolated-impurity picture to the high- density Thomas-Fermi description. Predictions for realistic Al_0.3Ga_0.7As/GaAs structures are given.

In this paper, the influence of growth rate on the dopant confinement of silicon delta-doped GaAs epitaxial layers grown by low pressure metal organic vapor phase epitaxy (LP-MOVPE) was studied in detail. It was found that the growth rate influences the dopant confinement of Si delta-doped GaAs layers in the case of low silane flows. In contrast, at higher silane flows, growth rate has very little effect on the carrier confinement. The Si delta-doped GaAs layers with very good confinement and high sheet carrier concentration have been achieved under optimal conditions of LP-MOVPE.

We present a theoretical study of hot-carrier induced light emission in III-V semiconductor devices. Carrier heating under the intense electric fields present under high bias conditions are studied via a selfconsistent Monte Carlo simulation. The carrier distribution functions obtained from the simulation are then incorporated into a pseudo-potential algorithm that describes the direct optical transitions and calculates the corresponding spectra. We show that the light emission due to hot carriers is dominated by direct radiative interband transitions within the conduction and valence bands. Good agreement between theory and experiment is obtained for GaAs MESFET and GaAs/AlGaAs HBTs.

In this paper, we characterize impact ionization effects in GaAs MESFETs, in AlGaAs/GaAs High Electron Mobility Transistors, HEMTs, in AlGaAs/InGaAs Pseudo-Morphic High Electron Mobility Transistors, PM-HEMTs, and AlGaAs/GaAs Heterojunction Bipolar Transistors, HBTs. Then, we correlate impact ionization and electroluminescence in each type of device, providing a deeper insight into the mechanisms responsible for the emission of photons in the high electric field regime. Finally, conclusions follow in Section 4.

GaAs layers as well as GaAs MESFET devices on InP are studied by means of DLTS and PL spectroscopies. We correlate the compensation observed on the Si-n-doped GaAs layers to the incorporation of Si that moves from a Si_Ga donor site to form a complex defect involving Si and As or Ga vacancies. A study of defects on MESFETs grown with various buffer layer thicknesses shows that the thicker this layer is the higher is the defect concentration. This behavior is assumed to be related to the compensation effect.

The method of picosecond laser induced grating (LIG) was used for investigation of non- equilibrium charge carriers (NCC) dynamics in the epitaxial layers of InGaAsP and InGaAs. The carriers recombination time (tau) _R and diffusion coefficient D_a have been determined. The influence of bleaching on the revealed values is discussed.

Planar In_XGa_1-XAs photodiodes with a cut-off wavelength of 2600 nm suitable for fiber optic and other applications at temperature of over 50 degree(s)C were the subject of this investigation. They were VPE grown on a sulphur doped InP substrate with an InAs_yP_1-y epitaxial graded layer structure in order to suppress the propagation of mismatch dislocations from layer to layer and to accommodate the large lattice parameter mismatch between the InP substrate and the In_0.88Ga_0.12As absorption layer. Deep- level-transient-spectroscopy (DLTS) was utilized as the main method in characterization of process-induced defects in the lower-doped n-type InGaAs absorber material close to the p-n junction. DLTS spectra indicate a broad distribution of energy levels corresponding to electron traps. The peaks of the distribution are located at 0.25, 0.30, and 0.42 eV below the conduction band. The DLTS spectra are pulse length dependent and the maxima shift to higher temperature with an increase of the pulse length, indicating that additional states are located deeper in the energy gap. Dislocations are the main candidates to be responsible for such a behavior. Minority carrier traps have also been detected at the InAsP/InGaAs heterojunction with an apparent energy level at E_v + 0.37 eV. Possible influences of the specific trap on the optoelectronic properties of these photodiodes are also discussed.

A systematic study of HREM images of AlAs/GaAs interfaces is presented. Special attention has led to the possibility of identifying steps at these types of interfaces. It is demonstrated that if a monolayer with a disordered alloy composition (Al_xGa_1-xAs exists between AlAs and GaAs, very small changes in the specimen thickness lead to contrasts which are similar to those originated for the existence of interfacial steps. A more detailed analysis than the visual inspection of the HREM images is required in order to get conclusions in relation to the presence of steps. The use of integrated profiles is a useful way to carry out this kind of analysis.

Rapid characterization of thin epitaxial Si_1-xGe_x layers on silicon substrate was made by multi-wavelength ellipsometry. Si_1-xGe layers were deposited by chemical vapor deposition in an atmospheric pressure reactor at 700 degree(s)C. A thickness variation of these strained thin layers (20 to 80 nm) was generated for different germanium contents in the range of 0.10 < X < 0.22. For ellipsometrical measurements a PLASMOS-ellipsometer SD 2000 at an incident angle of 70 degree(s) at different wavelengths was used. Theoretical simulations for measured ellipsometrical angles (Psi) and (Delta) and an iteration procedure were achieved to estimate the refractive index and thickness of Si_1-xGe_x layers on silicon substrate. The compositions are obtained by x-ray double crystal diffractometry to provide a correlation to ellipsometrical measurements. Stress in the thin layers is determined by Raman line shift. We found a relation between the real part of the refractive index of Si_1-xGe_x layers and the Ge-content in these layers at the wavelengths of 632.8 nm and 785 nm.

We show that there is a well defined class of semiconductor heterostructures (e.g., GaSb- InAs-AlSb, CdHgTe-CdTe) which do not obey the universal scaling law valid for below band gap excitations in bulk insulators and semiconductors. The magnitude, and the frequency at which the third order susceptibility changes its sign, are tunable over a range of energies spanning a significant fraction of the band gap, and reflect the separation in energy and the character of the dispersion in momentum space of higher lying minibands. The role of the microscopic (e.g., band structure) effects is also demonstrated in the range of photon energies capable of generating optical nonlinearities in the mid and far infrared, e.g., via excitations between valence minibands in p-type GaAs-AlAs quantum well structures.

A number of polydiacetylenes utilizing conjugated side groups containing heterocyclic rings have been made both as liquid crystals and as guest host systems. These materials exhibit promise for nonlinear applications.

An improved two-band model is proposed for excitons in GaAs-Ga_1-xAl_xAs quantum wells, where the x dependence of the anisotropic effective masses and the dielectric mismatch are both included. Exciton energies and oscillator strengths are obtained for typical asymmetric quantum wells. The mean field model of Agranovich is then used to derive the dispersion laws of the exciton polariton modes. The splitting of the mode polarized in the growth direction from the in-plane modes is in agreement with available experiments. We also compute the optical properties (transmission, reflection, absorption) of multiple quantum wells, and find new effects in the asymmetric case. Nonlinear optical properties of multiple asymmetric quantum wells are also considered, and it is shown that harmonic generation near the exciton resonance frequency is orders of magnitude higher than in bulk material for special geometries of the microstructures. This is in agreement with anomalously large values of (Chi) ⁽²) measured in double asymmetric wells.

A new, fast, intersubband 1.55 micrometers electro-optic modulator in the SiGe/Si/CaF₂-on- Si stepped-quantum-well system is proposed and analyzed. At applied electric fields of +/- 8 V/micrometers , resonant 1 - 3 conduction-intersubband absorption is predicted to given 18 dB of optical extinction for narrow-linewidth transitions. The procedure for incorporating conduction band nonparobolicity for higher lying subbands is described, and the issue of narrow linewidths is discussed.

Transmission of dye laser light pulses through GaAs/Ga_0.7Al_0.3As ridged waveguides, with propagation parallel to the Multiple Quantum Wells (MQW) layer, was investigated. Single-mode optical fibers have been used both to deliver light into the waveguides and to collect the transmitted pulses. The obtained results show the appearance of bistable cycles for input intensities above 300 kW/cm². Room temperature fluorescence was also investigated under 6328 angstrom irradiation observing HH1 and LH1 transitions.

Since within the effective mass approximation the Hamiltonian describing the electronic states of a semiconductor in the presence of crossed electric and magnetic fields contains non local potentials, then the interaction Hamiltonian between matter and radiation field has to be generalized in order to preserve the gauge independence of physical quantities. The two photon interband transition rate between two particular levels is evaluated in both length and velocity gauges demonstrating the correctness of our approach, moreover, the difference with the traditional approach is discussed.

A set of InP/GaInAs 30 periods superlattices with a wide range of barrier and well thicknesses were grown on (100) InP substrates by metal organic vapor phase epitaxy (MOVPE). Their optical response was measured at room temperature by spectroscopic ellipsometry from 1.8 to 5 eV. The nominal structures were checked by two methods of multi-layer modelling: full simulation using bulk dielectric functions for each layer and effective medium approximation, with the appropriate volume fractions. The thicknesses of well, barrier, and oxide overlayer were determined, and the effects of the interface region were discussed. The dielectric functions derived from the measured tg (psi) and cos(Delta) spectra were compared with the simulated ones obtained from the fitted structural model.

Spectroscopic ellipsometry has been used to determine composition and thicknesses of pseudo- morphic Si/Si_1-xGe_x multilayer structures. The effects of composition, strain, and quantum confinement on dielectric functions have been determined. The technique has been used to assess multi-quantum well structures with well thicknesses down to 20 angstroms.

Electroreflectance (ER) is a modulation technique which measures the change in reflectance of a semiconductor when an electric field is applied. Vacuum electroreflectance (VER), a new way of performing the experiment without making contact to the front surface of the sample, is briefly described. Gallium arsenide samples with carrier concentrations >= 10¹⁷ cm^-3 were studied by VER, electrolyte electroreflectance (EER), and photoreflectance (PR). Spectra obtained by these three techniques are compared, as are the results of fitting a new high-field ER theory to these spectra.

In this paper we report experimental results on InGaAs/InAlAs single quantum wells (SQW) obtained by photoreflectance (PR) between 5 K and 450 K. In the first part of the paper we focus on the evolution of the broadening parameter of E1H1 in the lattice matched 5 nm well width sample, E1H1 and E₂H₂ in the lattice matched 25 nm SQW. From this study we derive information about the relative influence of interface roughness, alloy scattering, and electron phonon interactions. In the second part we apply the PR technique to the study of quantum wells near the surface in which we observe an increase of the broadening parameter. These studies show the great interest of PR technique for the qualification of materials and for the surface probe.

The excitation dependent carrier recombination lifetime in periodically (delta) -doped strained InGaAs/GaAs multiple quantum well structures has been investigated both experimentally and theoretically. Experimentally, we find more than six orders of magnitude increase in the lifetime over that for undoped material due to the spatial separation of photogenerated carriers. This results in strong photo-optic effects and optical nonlinearities. Calculations, on the other hand, predict intrinsic recombination lifetimes in the periodically (delta) -doped material far above those found experimentally. Using secondary ion mass spectroscopy, transmission electron microscopy, cathodoluminescence imaging, and electron beam induced absorption modulation imaging we find evidence for misfit dislocation related recombination mechanisms that limit the carrier lifetime in the strained quantum well material.

One of the most interesting effects of nipi structures is the lifetime enhancement of excess carriers due to the built in bandedge modulation which separates the optically generated e-h- pairs in space and finally leads to an enhancement of the response. However, the lifetime enhancement generally slows down the detector speed. We present a nipi based detector concept which takes advantage of the enhanced response, but at the same time allows us to reduce the response time far below the characteristic lifetime in such structures. The physical background of the concept is presented in a theoretical model which can be understood in terms of the enhanced ambipolar diffusion and the local control of the balance of carrier generation and recombination by applying a lateral external current between two selective contacts. Results of numerical simulations are compared to experimental data obtained from selectively contacted PbTe single period (p-n-p) nipi-structures.

Recent research activity in the long wavelength infrared spectral region has been driven by the wide range of possible applications for optoelectronic systems and by the wealth of new physical phenomena displayed by quantum well intersubband-based devices. GaAs-AlGaAs quantum well intersubband photodetectors are currently under intense research investigation, and promise to have a wide range of application. Here we first present a microscopic model of the detector dark current, which provides new physical insights into the mechanism of carrier transport in multiple quantum well structures. Systematic experiments have been carried out to compare with the model, and good agreement between theory and experiments is obtained. The model is based on considering the details of the carrier trapping and emission from quantum wells.

Epitaxial narrow gap IV-VI layers on Si-substrates offer an alternate to Hg_1-xCd_xTe for infrared-focal plane arrays (IRFPA) with similar ultimate sensitivities. We report on the following improvements in reaching the goal of an easily producible fully monolithic IV- VI-on-active-Si IR-FPA: (1) Up to now, compatibility with the Si-substrate was reached through use of a stacked CaF₂/BaF₂ buffer layer, this layer is replaced by a very thin CaF₂ buffer only, which is more suitable for photolithographic processing; (2) fine resolution wet etching of the IV-VI layers is much easier and reproducible with this new type of buffer; and (3) etching of the surface was found to improve the behavior of the blocking Pb-contact towards better I-V characteristics.

Using 45 degree(s) geometry or grating for optical coupling, three kinds of GaAs/Al_xGa_1-xAs multiquantum well (MQW) photodetectors having different response range, i.e., 7.50 - 8.85, 8.20 - 9.80, and 9.60 - 11.40 micrometers , have been fabricated and evaluated. High responsivity Rp > 1 X 10⁵ V/W, high quantum efficiency (eta) > 14%, and high detectivity D_(lambda )^* > 3 X 10¹⁰ cmHz^1/2/W, were achieved at T equals 77 K.

A highly sensitive fast room temperature FIR detector is proposed. The detector operation is based on intraband photoconductivity in semiconductors. It has a time resolution of 100 ps and operates in the spectral range from 50 to micrometers to 500 micrometers . Its sensitivity (at (lambda) equals 90 micrometers is 30 (mu) V/W without and 6 mV/W with a 300 MHz bandwidth amplifier) increases with wavelength as (lambda) ². This is at least by two orders of magnitude higher than the sensitivities of fast pyroelectric or photon drag detectors.

A point fast room temperature detector for FIR laser radiation is proposed. Its operation is based on a new physical effect which consists of the stimulation of tunnelling during the plasma reflection of laser light in a Schottky-barrier tunnel junction.

Heterodiodes have been fabricated by PECVD growth of a-Si:H layers on crystalline silicon substrates. These diodes exhibit interesting properties for applications as photodetectors and solar cells. The impact of an argon plasma exposure of the crystalline silicon before the deposition of the amorphous silicon has been studied by in-situ microwave detected transient photoconductivity measurements and compared to the breakdown characteristics and the photoresponse of these heterodiodes. It has been found that the argon plasma pretreatment leads to a high interface recombination rate and influences the reverse bias characteristics of charge carrier collection in the photodiodes. The built-in potential for samples undergoing argon plasma treatment before a-Si:H deposition is reduced by 130 meV compared to the untreated samples.

Transient Reflected Microwave Conductivity (TRMC) measurements after laser pulse excitation have been performed during processing of thin film semiconductors. In particular PECVD-deposition of hydrogenated amorphous silicon (a-Si:H) and etching of the a-Si:H films in a hydrogen plasma has been studied by this non-invasive technique for the determination of excess charge carrier concentrations, mobilities, and lifetimes. During the growth of two intrinsic amorphous silicon layers deposited under different plasma conditions on top of each other, the comparison of the photoconducting properties of these layers already during deposition is shown and compared to conventional ex-situ characterization. By analyzing the interference fringes seen in the TRMC-amplitude during amorphous silicon growth and etching on crystalline silicon wafers covered by a dielectric layer an in-situ thickness control of the a-Si:H film has been achieved. Interface characterization in double layer structures with amorphous silicon deposited on top of crystalline silicon gave an estimation of the interface recombination rate and its change during the a-Si:H growth. An evaluation of the plasma induced damage in the initial stage of the silane plasma process is demonstrated. It has been found that already a very thin layer of a-Si:H inhibits the further defect creation at the surface of the crystalline silicon.

A new type of semiconductor photosensor has been suggested and designed. Its major feature is that the size and configuration of the photosensitive region on the working surface, as well as the value of its photosensitivity to registered light, are governed by control light flux. This allows us to use the new device in optical data processing systems for various important fields, such as image signal forming, determination of light spot center position, and others. The photodetector is based on a new effect of photoinduced reversible local electric field redistribution due to the space charge of free photocarriers. On the basis of field redistribution measurements the suitable semiconductor structures were found and the first samples of the light controlled photodetector were realized. They have demonstrated rather high detectability at the control light intensity about 10^-2 W/cm², spatial resolution up to 5 lines/mm, and speed of 10⁵ cycles/s.

The purpose of this work is to understand how the recombination of carriers generated by light (at several temperatures) can influence the a-Si:H devices quality. Here we report a comparative analysis between the photovoltaic performances of the PIN diodes and the transport properties of their active i-layers under the same degradation conditions.

A rectangular dual-axis large area position sensitive detector (PSD), with 5 cm X 5 cm detection area, has been developed by using a hydrogenated amorphous silicon (a-Si:H) PIN photodiode produced by plasma enhanced chemical vapor deposition (PECVD). The requirements needed for the fabrication of these devices are the thickness uniformity of the different layers, the geometry, and the contacts location. In this paper we present results on PSD with special emphasis on the linearity as well as on its response time.

Through anodically etching p-type Silicon in an ethanoic solution of HF, we prepared several porous samples whose physical properties have been investigated by different techniques, including photoluminescence, Raman scattering, Auger spectroscopy, secondary ion mass spectrometry, scanning electron microscopy and transmission electron microscopy. Schottky contacts on the porous surfaces have been deposited using different metals (Au, W, Ti and Al). In this way light emitting diodes have been obtained in the visible region. The performance and stability of the structures as well as the changes in the physical properties after the operation of the devices have been investigated. From the characterization of the physical properties and the behavior of the light emitting diodes a schematic band diagram of metal-p-Si structure is proposed and discussed. A tentative explanation of the different luminescence quantum efficiencies observed in p-Si for different excitation mechanism is given.

Strong time evolution of the porous silicon photoluminescence (PL) spectrum was first found and investigated. Two radiation bands were demonstrated under pulse optical excitation.

Time resolved luminescence measurements under resonant excitation were performed at 10 K on the free exciton in very high quality GaAs-AlAs quantum wells. At resonance, the luminescence intensity is increased by two orders of magnitude, and the decay is short. The radiative lifetime of the excitons in the radiant states is about 18 ps in agreement with the theory of Hanamura and Andreani. The lifetime of the thermalized excitons is about 80 ps at low excitation and it increases with the excitation density.

In this work, the experimental evidence of exciton confinement in the GaAs barriers of InGaAs/GaAs multiple-quantum-well structures is reported. This has been achieved by an ad hoc devised luminescence self-absorption spectroscopy method, as well as by standard photoluminescence, performed at different temperatures, which present a spectral feature at energies higher than those of bulk GaAs. The confinement energy and the linewidth depend on the barrier width, in agreement with a simple quantum mechanical model. Higher index states of the barrier exciton are also observed. The data underscore the critical importance of the choice of the sample-structure parameters for the confinement to be detectable.

We have experimentally studied the time-evolution of the exciton population in a higher subband of GaAs quantum wells, below the free carrier continuum. The lifetime of the exciton formed by an electron of the lowest subband and a heavy hole of the second subband in GaAs quantum wells is determined by time-resolved luminescence at 130 +/- 20 ps. This result is consistent with theoretical estimations of intersubband scattering by acoustic phonon emission. The exciton lifetime in the second heavy-hole subband is considerably longer than reported values of the recombination time in the lowest exciton state at k equals 0. The excitons in the higher subband at k equals 0 can be excited selectively without exciting the lower subband at k > 0. From these findings we conclude that subband transitions of excitons in quantum wells represent a new appealing concept for optically pumped coherent sources in the meV range.

A series of strained AlGaInP quantum well lasers has been fabricated with Ga_xIn_1-xP wells of a fixed width, L_z equals 65 angstrom, in which x was varied to give both compressive (x < 0.51) and tensile (x > 0.51) strain. By cooling these lasers to temperatures approximately 130 K we have been able to isolate the intrinsic effects of the strain from the extrinsic changes due to the quantum well structure and we see reductions in threshold current as compressive and tensile strain is applied.

Frequency tuning of a vertical-cavity surface-emitting laser (VCSEL) achieved through the monolithical integration of a modulator diode is reported. Current injection into the modulator diode locally changes the refractive index. This in turn leads to a shift of the Fabry-Perot- resonances of the microcavity. The experimental results show a gradient of a frequency blue- shift up to 0.93 GHz/mA by the modulator current. The maximum obtained frequency shift was 14 GHz at 15 mA modulator current. The useful tuning range is at present restricted to approximately 40 mA modulator current due to thermal effects. The onset of these effects leads to a bending over of the frequency shift from a Plasma effect dominated regime to a thermally dominated regime and in turn to a frequency red-shift. A simple theoretical model considering Plasma effect and Joule effect agrees well with the experimental data and predicts a maximum value for the gradient of frequency shift of 1.15 GHz/mA with the given structure.

The inclusion of tensile strain in the active region of long wavelength laser diodes has been shown to improve the device efficiency and alter the polarization output. It has been proposed that this is due to the effect of strain on the valence band structure and a subsequent change in the polarization selection rules. We have used uniaxial stress to simulate tensile strain in the active region of bulk 1.55 micrometers InGaAsP laser diodes and lattice matched 1.55 micrometers InGaAs multiple quantum well laser diodes. We observed an increase in transverse electric (TE) threshold current in both types of device, but with different rates of change in the lasing wavelength with stress. To understand the observed increase in threshold current we modeled the bulk device using a 4 X 4 Luttinger-Kohn Hamiltonian and then used this to qualitatively explain the change found in the quantum well device. The loss mechanisms of Auger recombination and intervalence band absorption (IVBA) were found to play a significant role in the increase in (TE) threshold current with applied stress.

Optical and electronic properties of lattice matched and strained quantum well laser media are analyzed for high carrier densities, combining many-body and band structure engineering techniques. The compressive strain yields more efficient inversion with consequent lower threshold carrier densities. The combined influence of strain and Coulomb effects on the (alpha) -factor causes a complicated dependence of (alpha) on the quantum well structure, laser frequency, carrier density, and threshold gain as observed in experiments.

A novel surface emitting laser diode beam steering device based on the excitation and emission of surface modes is presented. This new device is a longitudinally segmented wavelength tunable GaAs/AlGaAs double heterostructure laser diode, which is modified to allow a coupling of the laser mode to a transverse electric polarized surface mode. This results in surface emission with very narrow beam divergence. A steering of the surface emitted farfield pattern is achieved by a variation of the emission wavelength: By electrically changing the emission wavelength from 877.54 nm to 879.13 nm the dominant surface emitted peak is steered by 0.4 degree(s).

P- and n-type doping of ZnSe is the key technology to fabricate blue/green laser diodes. ZnCdSe/ZnSe quantum-well stripe-geometry laser diodes have been grown by molecular-beam epitaxy with nitrogen radical doping and chlorine doping techniques. Lasing was obtained in the pulsed operation at room temperature by applying high-reflectivity facet coating and in the continuous wave operation at 77 K. The characteristic temperature was obtained to be 137 K below room temperature. The cavity parameters were investigated at 77 K for a single- quantum-well separate-confinement-heterostructure. An internal loss of 1.2 cm^-1, an internal quantum efficiency of 61%, a transparency current density of 10 kA/(cm²micrometers ) and a gain factor of 0.095 cmmicrometers /A were obtained. The optical gain is improved by lowering the dislocation density in the laser structure.

We have predicted and confirmed experimentally that in InAs/InAs_1-x-ySb_xP_y system both type I and type II heterojunctions can exist. The occurrence of type II structure favors stimulated radiation on interface transition. In isotype p-InAs/P-InAsSbP heterojunctions with x equals 0.12 and y equals 0.25 a new interface line is observed which can be interpreted as an evidence of electron localization at the interface due to their over-barrier reflection off the boundary.

The optical properties of AlGaAs/AlAs/GaAs double barrier quantum well (DBQW) structures, obtained by the insertion of ultra-thin barriers of AlAs between the Al_xGa_1-xAs alloy and the GaAs wells, are investigated by means of continuous-wave and time-resolved photoluminescence techniques. Particular attention is devoted to the structure of the electronic states and to the recombination lifetime of the photoexcited carriers. We demonstrate the usefulness of this novel type of heterostructure as a further degree of freedom toward a tailoring of the band edges in GaAs/AlGaAs QW systems and also for the study of the carrier dynamics in the limit of very shallow subbands. Clear evidence of the transitions between 2D - 3D exciton behavior and between type I - type II recombinations are found depending on the thickness of GaAs and AlAs layers.

The role of surface states in reducing the radiative efficiency of a GaAs/AlGaAs quantum well (QW), situated in proximity of the surface, has been investigated. The near-surface QW photoluminescence (PL) was utilized as a probe of the effects of room-temperature hydrogen irradiation and of the subsequent evolution of the system in time. The e1 - hh1 PL at 1.4 K of various near-surface wells, differing in distance from the surface, was found to drop when the AlGaAs barrier was made thinner than 150 angstroms, due to short-circuiting recombination processes at the surface. The data were interpreted in terms of electron and hole tunneling to surface states. A study of the stability of the passivation effect -- samples being investigated again after an eight-month-long exposure to air, or after annealing in vacuum -- is indicative of important changes in the lifetimes of the different radiative and non-radiative processes associated with the well.

Electroreflectance and photocurrent measurements at low temperatures have been carried out in order to investigate Stark-ladder transitions in a GaAs(40 angstrom)/AlGaAs(20 angstrom) superlattice under various uniform electric fields, and compared with the transition energies calculated on the basis of a microscopic tight-binding description. The observed electroreflectance spectra in a wide range of energies (1.5 eV - 2.2 eV) shift in proportion to an applied electric field. The signals in the higher photon energy region (1.9 eV- 2.2 eV) indicate an existence of the transition from the spin-orbit split-off band in the valence band to the Wannier-Stark localization states in the conduction band. The assignment is supported by the tight-binding calculation.

The absorption-edge spectra have been studied in the temperature range from 4 to 300 K for single-crystal CdTe, ZnTe, GaAs wafers and GaAs/(Al,Ga)As multiple quantum well (MQW) structures. In all cases the frequency integrated absorption coefficient K is found to increase monotonously with temperature T up to T = T* and to keep constant above T*. The temperatures T*, depending on semiconducting materials, might be considered as critical ones corresponding to a change in polaritonic ener~ transport mechanism due to the lack of spatial dispersion at T<T*. It was shown, that though the measured temperatures T -. o2 K correspond to much larger linewidths than could be explained by using the theoretical value of the exciton damping parameter, this discrepancy can be overcome if a temperature-dependent inhomogeneous broadening is taken into account consistently. A similar temperature dependence of K with T* = 20 K has been observed for the first time in GaAs/( Al, Ga)As MQW-structures.

The exciton binding energy have been calculated as function of the magnetic field and the quantum well width for the various exciton and magnetic states. The variational calculation carried out in the adiabatic approach has shown the sizeable enhancement of the exciton binding energy with the increase of the external field. For rather strong magnetic fields the exciton binding energy has been found in the framework of the perturbation theory. The results were used for the analysis of the spectra of light transmission through the thin semiconductor film containing GaAs/Alo,3Gll(),7As multiple quantum well structure in the external magnetic field tuning from 0 to 7 .S T at T = 2 K.

Based on a frequency tunable twin-guide (TTG) InGaAs/GaAs multiple quantum well (MQW) laser structure, we developed a novel design concept for a surface emitting laser device enabling spatial beam steering. Utilizing a change in the refractive index of the parallel monolithically integrated modulator diode due to carrier injection, we observe a continuous emission frequency (wavelength) shift up to (Delta) f equals 85 GHz ((Delta) (lambda) equals -0.35 nm). For this preliminary structure the experimental results are consistent with our model calculations. Based on the theoretical model, for an optimized device a tuning range of (Delta) f equals 1600 GHz ((Delta) (lambda) >= 5 nm) is expected. For the novel surface emitting device design, we make use of an additional structure on top of the TTG laser including a second waveguide and a grating. This will enable a wavelength dependent surface emission angle, i.e., continuous beam steering, by coupling the laser and the surface mode. A calculational model was developed to estimate the steering characteristics in dependence on the dielectric device structure including mode guiding and the surface grating shape.

Semiconductor diode lasers emitting in the visible and near-infrared region of the spectrum are valuable sources of coherent radiation in several fields of fundamental and applied research. In particular, the properties of spectral purity and frequency tunability are major issues for their use in spectroscopy or in coherent communication systems. The emission characteristics can be improved using optical feedback from external cavities. A reduction of the emission linewidth by several orders of magnitude was achieved for visible and near-infrared lasers. Although the frequency modulation capabilities are usually affected by the presence of the optical feedback, we found that under special resonant conditions it is possible to have simultaneously a narrow-linewidth laser with a high-frequency modulation capability. Modulation frequencies up to several GHz were achieved. Applications of these devices to spectroscopy are described.

We present an experimental and theoretical study of the carrier capture time in a semiconductor quantum well. We observed for the first time the predicted oscillations of the phonon emission induced capture time experimentally and found good agreement with theory. Calculations show that not only the LO-phonon emission induced capture time (ph-capture) oscillates as a function of well width, but also the carrier-carrier scattering induced capture time (c-c capture) oscillates by more than an order of magnitude as a function of the active layer design. Moreover, the calculated amount of excess carrier heating also oscillates as a function of quantum well thickness. Recently, it has been shown that the carrier capture time is directly related to the nonlinear gain in a quantum well laser. As a result, the nonlinear gain can be tailored by optimizing the capture efficiency using a proper design of the active layer in a quantum well laser.

The thermal behavior of a diode laser is studied through the photodeflection method. A new method is proposed to detect the cooling property of laser diode, during its operation. A theoretical model is presented together with some experimental results obtained on AlAs-GaAs lasers.

The bistable and modulation characteristics of MQW devices with the same structure were measured. It still keeps the emission from CCTS laser with stable single mode. The modulation depth of 22.4 dB under 2 V bias was obtained. The integration consideration was suggested.

Optically stimulated luminescence (OSL) of gamma-irradiated undoped KMgF₃ showed emission bands at 464 and 534 nm. Incorporation of Gd resulted in new emission bands around 275 and 310 nm. In RbMgF₃ four emission bands at 302, 400, 465, 800 and shoulders at 525, and 770 nm are observed, respectively. The strong emission band of Gd at 311 nm is suppressed significantly in OSL process of RbMgF₃. The presence of Gadolinium in CsMgF₃ suppressed the 300 nm band seen in CsMgF₃ and resulted in a strong emission around 312 nm. The OSL emission spectra of LiBaF₃ consists of three emission bands and a shoulder at 385, 470, 535, and 415 nm, respectively. Doping of Europium in LiBaF₃ suppressed the OSL emission of LiBaF₃ and resulted in the characteristic emission of Eu²⁺ at 360 and 414 nm. The OSL spectra of these samples have been compared with their thermally stimulated luminescence (TSL) spectra and the results are discussed in terms of the formation of F and V_k-centers. The disappearance of certain bands has been attributed to the energy transfer processes between the activator and host.

PMMA Poly(methyl methacrylate) is doped with a photochromic fulgide. The fulgide undergoes a reversible refractive index change at UV irradiation. This colored form can be bleached by irradiation with a wavelength in the visible. The photoinduced refractive index change (Delta) n is studied for various concentrations and UV intensities by holography. The tunable effective refractive index can be used for controlling the intensity of a guided mode in thin film waveguides (0.5 - 5 micrometers ) by an external UV light source. Monomode waveguides are used as a cut-off modulator (opto-optical switch). The switching time is about 20 ms.

The enhancement of the refractive index n((omega) ) close to interband transitions between confined quasi two-dimensional states of the valence and conduction bands of PbTe/Pb_1-xEu_xTe multiquantum well structures (MQWs) is studied experimentally as well as theoretically. The cusp-like changes of n((omega) ) are of importance for the proper design of quantum well lasers. We use transmission and reflectivity data of several MQWs with a well to barrier thickness ratio of about 4 - 10 which were deposited on 3 to 5 micrometers thick Pb_1-xEu_xTe buffers grown on BaF₂ substrates for a comparison with calculated spectra.

The paper describes the development of an optical microphone on an integrated-optical basis. The applied modulation principle is based on the change of the coupling ratio between two butt-coupled optical waveguides induced by lateral displacement of the guides. The coupling beam is first directed to a reflective membrane before being coupled into the receiving waveguide, allowing the membrane movement to be detected in a non-contact manner. The necessary waveguide structure is contained in an integrated-optical chip. This chip was produced with the aid of a technique based on the polymer PMMA, specially developed for this purpose. The necessary change of the refractive index was generated by means of a photochemical method. The coupling performance of the chips is largely in agreement with the theoretical values.

Thin polyimide films, prepared by vacuum deposition, have been investigated. The influence of the nature of the monomers, evaporation rate, treatment of the substrate and post- preparation treatment (imidization) of the layers on the structure are discussed. Infrared spectra and thermogravimetric analysis results are consistent with the interpretation that VDP films produced are polyamic acid curable to polyimide. The morphology and properties have been characterized by electron microscopy.

We report a novel method for growth and fabrication of high performance strained AlGaAs/InGaAs quantum well buried heterostructure (BH) lasers. The method involves growth of the laser structure by molecular beam epitaxy, mesa formation by in-situ melt etching using SiO₂ stripes as a mask, and regrowth of p p n AlGaAs isolating layers by liquid phase epitaxy. The method allows etching, preservation of high quality sidewalls, regrowth and planarization in one step with negligible thermal disordering. Compared to ridge wave guide (RWG) lasers, the BH lasers so fabricated have significantly lower threshold current, higher power output, higher temperature operation, lower cavity losses, and kink free light-current (L-I) characteristics, as expected. A cw power of 150 mW/facet at 986 nm was measured from a 400 micrometers long BH laser with 11 micrometers active stripe width. A minimum threshold current of 2.5 mA was measured for lasers with 3.0-micrometers active width and 300 - 400 micrometers cavity length. The L-I characteristics of 500-, 800-, and 1300-micrometers long lasers with 3.0 micrometers active width were linear up to the currents corresponding to a current density of 10 kA cm^-2. At higher current densities, a sublinear increase of power with current was observed. Stable fundamental transverse mode operation was obtained up to 100 mW emitted power.

We tune a two beam interferometer with a rotating linear polarizer using Pancharatnam's theorem.

Saturated and unsaturated fatty acids and mixtures of fatty acids and fatty acid-cholesterol in electric and optic fields were investigated. In the mesomorphic state, they are nonlinear dielectrics and nonlinear optical media. These properties are influenced by cholesterol.

A large class of new layered semiconductors -- perovskite lead iodide compounds -- is of great interest because of possible optoelectronic applications due to pronounced excitonic effects. It has been predicted earlier that excitons in these structures should be enhanced due to the so called `dielectric confinement' arising from large differences of dielectric constants of adjoining layers. In the present work we calculate the energies, the wave functions, and the diamagnetic coefficient of excitons in these naturally grown superlattices with allowance for the image potential and the superlattice structure of the compounds. The fitting of our theory to the experiments makes it possible to evaluate the reduced mass of excitons in these compounds.

We propose a new approach to the intensity modulation in antiresonant reflecting optical waveguides. The proposed mechanism involves a variation of the reflectivity of the interface between the cladding layers, obtained by providing a small refractive index difference between the core and the second cladding layer.

KTP crystals doped with rubidium (Rb) or similar ions are potentially valuable in the fabrication technology for active optical waveguides and other integrated optical components. The optical transparency, damage threshold, electro-optic coefficients and nonlinear properties suggest that it could become a material of preference. Simple waveguides and domain-reversed devices have already been tested. The composition and structure of the mixed crystals Rb_xK_1-xTiOPO₄, formed in the ion-exchanged waveguiding layer, is not well understood. In this paper the penetration depth of Rb and barium (Ba) from a mixed Rb/Ba exchange melt into the crystal is explored by SIMS, and by proton and electron microprobe. Rb is shown to diffuse deep into the crystal to form an exponential or semi-Gaussian concentration and refractive index profiles depending on the processing conditions. Ba is found to facilitate the Rb penetration but could be detected only at the surface of the waveguiding layer at the end of the process. Fabrication of flux-grown mixed crystals Rb_xK_1-xTiOPO₄ and comparison with a fully doped Rb:KTP sample of the same composition enabled an important structural difference related to the sites of the Rb ions to be noticed. The use of x-ray techniques has indicated that the ion-exchange process induces considerable strain in the surface layer, possibly with significant implications for active device performance.

We have used spectroscopic ellipsometry to investigate the optical constants of undoped ZnSe, ZnS_xSe_1-x and Zn_1-xCd_xSe films grown by molecular beam epitaxy on GaAs. Ellipsometric modelling has allowed access to the above and below bandgap optical constants of the films. From the results given, shifts in bandgap are obtained with variation of S or Cd content. The use of a suitable dispersion equation has allowed the determination of the below-bandgap refractive index of the ZnSe films involved. Our results for the optical behavior of ZnSe films are in reasonable agreement with data obtained from a simplified model of interband transitions for single crystalline non-doped ZnSe bulk material available in recent literature.

We used optical pumping by the Vanderbilt Free Electron Laser and the technique of internal photoemission to measure with high accuracy the conduction band discontinuity of semiconductor heterojunction interfaces. The experiment is the first application of a free- electron laser to interface research.